Chaldeans, or Kasdim. At first a tribe, then a caste of learned Kabbalists. They were the savants, the magians of Babylonia, astrologers and diviners. The famous Hillel, the precursor of Jesus in philosophy and in ethics, was a Chaldean. Franck in his Kabbala points to the close resemblance of the “secret doctrine” found in the Avesta and the religious metaphysics of the Chaldees.
Source: H.P.Blavatsky - The Theosophical Glossary